1. Field of the Invention
The present invention relates to an image forming apparatus such as a laser printer.
2. Description of Related Art
Laser printers and other image forming apparatuses mainly include: a photosensitive drum, a developing roller, and a transfer roller. The photosensitive drum is formed with an electrostatic latent image on its outer peripheral surface. The developing roller is disposed in confrontation with the photosensitive drum. The developing roller supplies developing agent, such as toner, to the photosensitive drum, thereby developing the electrostatic latent image into a visible image. The transfer roller is disposed also in confrontation with the photosensitive drum. The transfer roller is applied with a transfer bias voltage with a polarity opposite to that of the photosensitive drum.
Especially in non-contact type printers, a charger uniformly charges the outer peripheral surface of the photosensitive drum. A laser generating unit modulates a laser beam based on image data, and scans the laser beam across the outer peripheral surface of the photosensitive drum. As a result, a corresponding electrostatic latent image is formed on the surface of the photosensitive drum. The developing roller conveys, on its surface, toner that is electrically charged to the same polarity as that of the photosensitive drum. The electrostatic latent image on the photosensitive drum is developed into a visible toner image with the toner supplied from the developer roller according to a well-known reversal development process. The thus developed visible image is then transferred from the photosensitive drum onto a sheet of paper that is passing between the photosensitive drum and the transfer roller. The visible image is pulled onto the sheet of paper by an electrostatic field that is generated by the transfer bias applied to the transfer roller. Thus, one image forming cycle is completed.
According to the above-described image forming cycle, some toner remains on the surface of the photosensitive drum after the toner image has been transferred from the photosensitive drum onto the sheet of paper. According to a well-known cleanerless method, this residual toner is collected during the next image forming cycle. Thus, in each image forming cycle, development and cleaning are performed simultaneously by the developing roller according to reversal development process.
According to this cleanerless method, there is no need to provide a blade or other type of cleaner device in the image forming apparatus. There is also no need to provide a vessel to accumulate waste toner. Accordingly, configuration of the entire image forming apparatus can be simplified and made more compact. The image forming apparatus can be produced less expensively.
It is noted that when the sheet of paper passes between the photosensitive drum and the transfer roller, paper dust clings to the surface of the photosensitive drum. This paper dust will be possibly collected together with the residual toner. When the toner is reused during a later development process, the paper dust can degrade the resultant visible image. When an acid type sheet is used as the sheet of paper, the paper dust includes filler material such as talc. The filler material can cause filming and so magnify the problem of the defective visible images.
In view of the above-described problems, there has been proposed that the cleanerless-type image forming apparatus be provided with a paper-dust removing device such as a brush. The paper-dust removing device is positioned in contact with the photosensitive drum in order to remove the paper dust that clings to the photosensitive drum.
However, because the paper-dust removing device is in contact with the photosensitive drum, the residual toner also clings to the paper-dust removing device together with the paper dust. This will reduce the ability of the paper-dust removing device to remove the paper dust. The toner clinging to the paper-dust removing device can be smashed into the surface of the photosensitive drum, thereby generating filming of toner on the surface of the photosensitive drum.
There have been proposed several types of paper-dust removing device such as: (1) a rotational brush roller; (2) another rotational brush roller whose constituent brush fibers are formed in loops; and (3) a rotational non-woven fabric roller that includes a rubber roller covered with a non-woven fabric. The rotational brush roller (2) is disclosed in Japanese patent application publication (kokai) No.HEI-1-116677), and the rotational non-woven fabric roller (3) is disclosed in Japanese utility model application publication (kokai) No.SHO-62-181973.
However, these devices (1) through (3) have the following problems.
Devices (1) and (2) are able to properly remove fibers included in the paper dust. However, the brush in these devices is unable to sufficiently remove filler material such as talc in the paper dust from an acidic paper. As a result, talc will possibly be collected together with residual toner on the developing roller. The collected talc will generate an undesirable fogging phenomenon during subsequent development processes. That is, when an electrostatic latent image is developed with toner mixed with talc, fogging will be formed on the white areas of a developed image and as a result the image will be poor.
Device (3) is designed to strongly press the non-woven fabric against the photosensitive drum in order to properly remove the filler material such as talc from the surface of the photosensitive drum. However, the non-woven fabric will scrape the paper dust across the surface of the photosensitive drum. As a result, the soft talc is spread across the surface of the photosensitive drum, resulting in filming of talc on the photosensitive drum surface. The performance of the photosensitive drum will deteriorate.